1. Field of the Invention
This invention relates in general to subsea well drilling and production riser strings, and in particular to an emergency disconnect system that disconnects the lower end of the riser from the well during excessive bending and/or tension in the event of an emergency.
2. Description of the Prior Art
In offshore oil well drilling, a floating vessel will be used. The well at the sea floor connects to the vessel by means of a riser. The vessel may be anchored by anchors and other tensioning means over the well or wells or it may be a dynamically positioned vessel. Wave, wind and current can cause bending of the riser due to movement of the vessel. The riser is designed to withstand a certain amount of bending and tension that will be encountered during normal conditions. If an emergency condition arises due to a severe storm or other unusual circumstance, such as loss of anchors or station keeping ability of a dynamically positioned vessel, the riser may undergo excessive tensioning when the tensioners bottom out. The riser may part at some point along its length. This could result in major equipment damage and serious problems in retrieving and installing the riser again.
Riser strings connect to the lower marine riser package by hydraulic wellhead connectors. The wellhead connectors normally employ some type of gripping member, such as dogs, that are pushed by a cam ring radially inward into engagement with grooves formed on the exterior of the wellhead housing. Hydraulic cylinders move the cam ring axially to lock and release the gripping members. In the event of a storm, the operator could release the lower end of the riser from the blowout preventer stack if the bending or tension forces become excessive. However, the operator may not realize when the forces become excessive, therefore the riser string could part before the releasing occurs. Also, the release mechanism might fail.
Weak links have been proposed for riser systems. Generally the proposals involve employing frangible members that hold two of the sections together at a point near the lower end of the riser. The frangible members are selected to part at a lower axial force than any other portion of the riser. A drawback, however, is that conditions less than a true emergency may result in the frangible members parting unless they have a very high strength. If the strength is too high, damage could occur to the riser before the weak point parts.